Biomedical Engineering Reference
In-Depth Information
Accordingly, movement of the middle plate, which will change the initial distance,
d,
by
D
d
, will change the distance between two adjacent plates such that one capacitor will
increase while the other will decrease in value. This double-capacitor arrangement provides
improved sensitivity and can be incorporated into a Wheatstone bridge configuration.
Capacitance sensors can be mass-produced using solid-state microfabrication techniques
that are commonly employed in making integrated circuits.
EXAMPLE PROBLEM 10.9
Two metal plates with an area of 0.4
10
4
m are used
to form a capacitance transducer. If the material between the two plates has a dielectric constant
e
r
¼
10
3
m
2
and separation distance of 1
2.5, calculate the capacitance of the transducer.
Solution
e
r
A
10
12
F
10
4
m
2
10
4
m
C
¼
e
o
d
¼
8
:
85
=
m
2
:
5
4
=ð
1
Þ¼
0
:
885 F
Capacitive displacement transducers can be used to measure respiration or movement
by attaching multiple transducers to a mat that is placed on a bed. A capacitive displace-
ment transducer can also be used as a pressure transducer by attaching the movable plate
to a thin diaphragm that is in contact with a fluid or air. By applying a voltage across the
capacitor and amplifying the small AC signal generated by the movement of the dia-
phragm, it is possible to obtain a signal that is proportional to the applied external pressure
source.
Piezoelectric Transducers
Piezoelectric transducers are used in cardiology to listen to heart sounds (phonocardiog-
raphy), in automated blood pressure measurements, and for measurement of physiological
forces and accelerations. They are also commonly employed in generating ultrasonic waves
(high-frequency sound waves typically above 20 kHz) that are used for measuring blood
flow or imaging internal soft structures in the body.
A piezoelectric transducer consists of a small crystal (e.g., quartz) that contracts if
an electric field (usually in the form of a short voltage impulse) is applied across its
plates, as illustrated in Figure 10.17. Conversely, if the crystal is mechanically strained, it
will generate a small electric potential. Besides quartz, several other ceramic materials,
such as barium titanate and lead zirconate titanate, are also known to produce a piezoelec-
tric effect.
The piezoelectric principle is based on the phenomenon that when an asymmetrical crys-
tal lattice is distorted by an applied force,
, the internal negative and positive charges are
reoriented. This causes an induced surface charge,
F
on the opposite sides of the crystal.
The induced charge is directly proportional to the applied force and is given by
Q,
Q
¼
k
F
ð
10
:
13
Þ
